Process and apparatus for preparing sulphite digestion liquor



D. B. DAVIE-S AND E. P. STRONG.

PROCESS AND APPARATUS FOR PREPARING SULPHITE DIGESTION LIQUOR.

U APPLICATION FILED JUNE 1 192 0. I 1 424. 3 I PatentedAug. 8, 1922.

w u v Egg warren stare IDA B. DAVIES AND EDWARD P. STRONG, QF MUNISING, MICHIGAN.

ERUCESS AND APPARATUS .FQR PREPARING SULPHITE DIGESTTOII LIQUOR.

inaasee.

Specification of Letters Patent.

Patented Aug. d, 1922.

Application filed June 1, was. Serial no. esa'zos.

To all whom it may concern:

Be it known that we, DAVID B. DAVIES and EDWARD P. STRONG, both of Munising, county of Alger, and State of Michigan, have invented certain new and useful Processes and Apparatus forPreparing Sulphite Digestion Liquor, of which the following is a specification.

The invention relates to thesulphite di-,

gestion offibrous material and seeks to pro vide an improved method and apparatus for preparing the sulphite liquor and for recovering the sulfurous acid gas relieved from the digester.

For the manufacture of cellulose from woody or other fibrous material by the sulphite method, so called raw acid is usually first prepared by treating-lime water with sulfurous acid gas. This raw liquor ordinarily contains about the proper amount of combined sulfurous acid but less than the amount of free acid required in the finished or so called cooking acid. The finished digestion liquor is prepared by the in troduction into the raw liquor of gas relieved from the digester or cooking vessel and which usually contains about 95 to 98 per cent of sulfurous acid. The method now generally employed is to relieve the gas from the digester under pressure and, after passing the gas through suitable separating and cooling means, it is injected through submerged pipes into the sulphite liquor. The latter absorbs part of the gas and the proportion of free acid in the. liquor is increased. Any unabsorbed gas is returned to the plantfor the preparation of raw acid.

With this method, the gas in the relief system is necessarily under considerable pressure since it must overcome the hydrostatic head in the tank containing the sulphite liquor into which the gas is injected, and this pressure interferes with the comv lete separation of the gas from the waste liquor and condensed water vapor in the relief system. Again the injection of the gas agitates the li nor in the tank so that a considerable portlon of the gas passes oil unabsorbed and it is diiiicult to retain in the liquor the desired high proportion of free sulfurous acid.

In accordance with the present improved method, the liquor to be charged with the sulfurous acid gas, is circulated through an aspirator and thence through a long, preferably vertical pipe extending downwardly from the aspirator and opening into the lower portion of a storage tank. .The gas lower its temperature, is conducted to the=- aspirator and is drawn thereby into the confined current of sulphite liquor. Because of the practical purity of the sulfurous acid gas and since, by the operation of the as pirator, it is intimately mixed with the confined current of liquid, a rapid and practically complete absorption of the gas is effected and the gas can be relieved from the digester under less than atmospheric pressure into vacuum produced in the entire system between the aspire-tor and the digester relief pipe.

On this account, there is a much more complete separation of the sulfur dioxide gas from the waste liquor and condensed water vapor in the relief system than is the case where the gas is relieved from the digester against: pressure in the separator. Again practically all of the sulfurous acid gas is absorbed by the liquor before the latter enters the storage tank so that there is little agitation or bubbling in the tank. As a result, a more perfect retention of the gas is efi'ected and the free sulfurous acid in the liquid under treatment is more readily increased or built up to the desired relatively high percentage than is the case with other methods now employed.

The accompanying drawing illustrates diagrammatically an improved apparatus for carrying out the described process.

Thedigester or cooking vessel one, in which charges of wood chips or other fibrous material are treated with digestion liquor is or usual construction, and hasreliet' and blow-off pipes 2 and 3 that are connected respectively to its upper and lower ends an provided with valves 4 and 5. During each digesting operation, gas, water vapor, and some waste liquor are passed oil from the digester through the relief pipe and,

finally the waste liquor and cooked fiber are discharged through the pipe 3.

liquor trap 7. To prevent ebullition in the trap and safely seal the same, suitable cooling means, such as a cold water supply pipe 8 having a controlling valve 9, is provided. From the upper portion of the separator the gas is conducted by a pipe 10 through. a suitable cooler 11.

'A storage tank 12,. for receiving the sulphite liquor from the raw acid plant is connected by a pipe 13 to a centrifugal pump 14. The latter withdraws the liquor from the tank and forces it through an upwardly extending'pipe that is connectedto the annular chamber-of an aspirator 16. The gas supply pipe 11 is connected to the aspirator nozzle 17 and a return pipe 18 extends downwardly from the aspirator and opens into the storage tank 12 at a point near the bottom thereof. The usual pipes (not shown) are provided for conducting liquor from the raw acid plant to the tank 12, for returning any gas escaping from the liquor in the tank to the acid plant, and for conducting the finished cooking acid from the tank to the digester.

As the gas is relieved from the cooking vessel during each digestion operation, the pump 14 is actuated to withdraw liquor from the tank 12 and circulate it through, the pipe15, aspirator 16 and return pipe 18 back to the tank. The highly concentrated sulfur dioxide gas is drawn into the confined current of circulating liquor and is rapidly absorbed thereby so that the re, lieved gases can be maintained'under good vacuum to effectively separate the sulfur dioxide gas from the relief waste liquor and condensed water vapor. As stated the absorption of the sulfur dioxide is practically .40 complete before the liquor is returned to the tank. For this reason and since there is no appreciable quantity of other gases, there is little agitation or bubbling in the tank to prevent the retention of a high proportion of free sulfur dioxide. With the improved process and apparatus, it is,believed that the separation of the gas from' the relief waste liquorand condensed water vapor and its absorption by the fresh digestion liquor are much more complete and rapid than with other methods and apparatus heretofore employed.

The details set forth of the improved and portions thereof adopted without departure from the essentials of the invention as defined in the claims.

We claim as our invention: I

1. The process of preparing sulphite digestion liquor, which consists in withdrawmg the liquor from a storage tank, circulating the same in a confined current and returning it to the tank at a point below the surface of the liquor therein, and aspirating process and apparatus for the preparation .1

. thereby, substantially as described.

mamas? and water vapor from the gas, cooling the gas, and then, by aid of a confined current of sulphite liquor, aspirating the gas into the liquor to effect the absorption of the gas thereby.

4. In the sulphite di estion of fibrous material, the process 0 preparing cooking liquor and recovering the concentrated sul- -furous acid gas, relieved from the digester,

which consists in withdrawing the liquor from a suitable storage tank, circulating the same .in a confined current and returning it to the tank at a. point below the level 'of' the liquid therein, separating waste liquor from the gas and cooling the same, and then by means of the current of liquor, aspirating 1 the gas into, the liquor and thereby effecting the absorption of the as.

5. In the sulphite digestion of fibrous material, the process which consists in relieving the gas from thedigester and aspirating the same into a confined current of suI- phite li nor to thereby maintainthe relieved gas un er less than atmospheric pressure and effect the absorption thereof by the liquor. I a

6. An apparatus for preparing sulphite digest ion liquor comprising a storage tank for the sulphite liquor, apump and pipe connection for withdrawing the liquor from said tank, circulating the same in a confined current and returning it to the tank at a point near the bottom thereof, a supply. pipe for sulfurous acid gas and an aspirator interposed in said pipe connections arsubstantially as. described.

7-. In a sulphite digesting apparatus, a

digester, 'a gas relief pipe for said. digester,

means for separating waste liquor from the I gas passing through said pipe and for cooling thesame, an aspirator connected to said pipe, a storage tank for sulphite liquor and a pump and'pipe connectionv for circulating the liquor through said aspirator and into said tank at a point below the level of: the liquid therein, whereby the gas'is drawn by suction. into the liquid and is absorbed mamas v V g 8. In a sulphite digesting apparatus, a said digester to said aspirator and means 10 digester, a storag tank for sulphite liquor, for separating Waste liquor from the gas a pump for withdrawing liquor from said passing through said pipe connections and tank, a vertical delivery pipe for said pump, for cooling the same, whereby the gas is an aspirator connected to said pipe, a vertidrawn by suction into the liquor and is abcal return pipe leading from said aspirator sorbed thereby, substantially as described. 15 and opening into said tank at a point near the bottom thereof, pipe connections for de- DAVID B. DAVIES. livering sulfurous acid gas relieved from EDW-D P. STRONG. 

